A follow-on flight, expected the week of 24 May, is to see X2 test pilot Kevin Bredenbeck accelerate the LHTEC T800-powered fly-by-wire pusher to 180kt, completing the third of four planned envelope expansion test phases. Phase four, which could begin as soon as mid-June, includes the first attempts to slow the X2's main rotors while opening the speed envelope to 250kt or more.

The prototype is designed with no clutch between the main rotors and propulsor, which requires the pilot to increase forward speed through the variable pitch control on the six-bladed rear propeller. Once in the 180kt realm, the X2's computer will automatically slow the main rotors and increase collective pitch to prevent tip speeds from entering high-drag transonic region, with Bredenbeck correspondingly increasing propulsor pitch to increase the X2's speed as the propulsor also slows.

Recent progress includes flights with the main landing gear retracted and two of the three main rotor fairings attached, tests of the main rotor active vibration system and 20-30kt of sideways flight in winds as high as 17-20kt. "We have quite a bit more control power left in this rotor for side flight," says Steve Weiner, chief engineer for the X2 programme.

Jim Kagdis, Sikorsky's manager of advanced programmes, says the flight-test team will "try to go as fast as we can" before installing the centre rotor fairing to determine the effect of the device on drag.

Kagdis expects that when the centre fairing is combined with the two elliptical main rotor fairings, already installed, the X2 will see a hub drag reduction of 40-45% compared with no fairing.

Vibration effects, which were problematic on Sikorsky's previous XH-59A demonstrator in the 1970s, have been "extremely low" so far, even without the active damping system turned on.

"We're really waiting to see what we get when go faster," says Weiner, adding that tests to date indicate that the X2 will meet its vibration targets - about the level of a traditional helicopter flying at 140kt - when cruising at its top speed of 250kt with the active system engaged.

Weiner says the only aerodynamic change to the structure made during the first three phases of the programme was the addition of 0.28m² (3ft²) of area to each of the outboard vertical end plates for yaw stability.

Weiner credits configuration maturity to Sikorsky's advanced analytical tools given that the company did not perform windtunnel tests of the design before flight-testing. He says windtunnel tests would likely precede the design for a production model, the first of which could be a systems development and demonstration vehicle for the US Army's potential competition to replace the Bell OH-58D Kiowa Warrior later this decade.

Sikorsky plans to begin using a 250kt Cessna Conquest twin turboprop for chase aircraft operations on the next flight.